This invention relates generally to systems utilizing a beam to scan a record medium, and, more particularly, to systems for controllably adjusting a prescribed characteristic of the beam relative to the record medium.
Systems of this particular type are of special use in video disc playback machines of a type having tracking apparatus for maintaining a beam of light aligned with a selected track on a video disc. An example of such a machine and of one typical prior tracking apparatus is disclosed in U.S. Pat. No. 4,332,022, issued May 25, 1982, entitled "Tracking System For Player", filed Mar. 17, 1980 in the names of Ludwig Ceshkovsky et al. and assigned to the assignee of the present application now U.S. Pat. No. 4,332,022.
The apparatus disclosed in that application directs a reading beam of light via a radially-movable tracking mirror onto a selected one of a plurality of substantially circular and concentric recording tracks on the disc. The beam is reflected in accordance with the recorded information, and the intensity of this reflected beam is detected and suitably processed, to produce a tracking error signal representative of the deviation of the reading beam from the centerline of the selected track. This error signal is coupled back to the tracking mirror, to form a servo for controllably positioning the point of impingement of the beam on the disc in alignment with the centerline of the selected track.
Alternative tracking apparatus are disclosed in U.S. Pat. No. 4,232,201 to L. S. Canino, entitled "Dithered Center Tracking System", and U.S. Pat. No. 4,232,337 to J. S. Winslow et al., entitled "Method and Apparatus for Tracking an Optically Readable Information Track". In both of the disclosed apparatus, a reading beam of light is controllably positioned relative to a selected track on a video disc by a tracking error signal that varies in accordance with the position of the beam relative to the track centerline.
Although the tracking systems described above have proven generally satisfactory in maintaining a reading beam aligned with a selected track on a video disc, they have not been entirely effective when the disc includes defects such as dimples in its information-bearing surface. Dimples are believed to be caused by minute particles being lodged beneath a thin, metallic stamper used in molding the video discs, and they generally cause large groups of adjacent recording tracks to deviate momentarily from their normal radial locations.
Prior tracking apparatus have normally attempted to maintain the reading beam continuously aligned with a selected track, even when scanning in the vicinity of a dimple. However, because of the particular magnitude and duration of the track deviations caused by each dimple, this attempt at continuous alignment sometimes has caused the beam to overshoot the selected track and inadvertently jump to an adjacent track. When the reading beam is scanning a track on the approaching side of a dimple, this track-jumping phenomenon causes the beam to jump back repeatedly to the previously-scanned track. On the other hand, if the reading beam has passed through the center of a dimple and is scanning a track on its far side, this track-jumping phenomenon causes the reading beam to jump to the next succeeding track so that information stored on at least one track is not recovered.
It should be apparent to those knowledgeable in the field of scanning beam control systems, and particularly the field of scanning beam tracking systems, that there is a definite need for a system that can detect and compensate for deviations from normal operation of the specified characteristic being controlled. In particular, it should be apparent from the foregoing that a specific need has existed for a tracking system that includes means for detecting and compensating for record medium defects that otherwise can cause a loss of tracking. The present invention fulfills these needs.